Technological developments and increased demands for mobile devices have led to a rapid increase in demands for secondary batteries as energy sources. Among various secondary batteries, lithium secondary batteries having high energy density, high voltage, long cycle life, and low self-discharge rate are commercially available and widely used.
Moreover, as interest in environmental issues increases, there is growing interest in electric vehicles and hybrid electric vehicles being capable of replacing fossil fuel powered vehicles, such as gasoline vehicles and diesel vehicles, which use fossil fuel that is one of the main causes of air pollution. Accordingly, researches for using lithium secondary batteries as power sources of electric vehicles, hybrid electric vehicles, and the like are being actively conducted.
In order to use lithium secondary batteries for electric vehicles, the lithium secondary batteries should have high energy density and characteristics of generating high power in a short time, and also withstand over 10 years under severe conditions. Therefore, significantly better stability than typical compact lithium secondary batteries and long-term life characteristics are necessarily required.
A lithium secondary battery refers to a battery which includes an electrode assembly and a non-aqueous electrolyte containing lithium ions, wherein the electrode assembly includes a positive electrode including a positive electrode active material enabling intercalation and deintercalation of lithium ions, a negative electrode including a negative electrode active material enabling intercalation and deintercalation of lithium ions, and a microporous separator disposed between the positive electrode and the negative electrode.
Examples of positive electrode active materials of lithium secondary batteries include transition metal oxides such as a lithium cobalt oxide (LiCoO2), a lithium manganese oxide (LiMn2O4), or a lithium nickel oxide (LiNiO2), and composite oxides in which transition metals contained in each aforesaid material are partially substituted with other transition metals.
Among the positive electrode active materials, LiCoO2 is being widely used due to its excellent overall properties such as cycle characteristics. However, LiCoO2 has low stability and is costly due to resource limitations of cobalt as a raw material, thus disadvantageously having limited mass-utilization as power sources in the fields of electric vehicles or the like.
Lithium manganese oxides such as LiMnO2 and LiMn2O4 advantageously are abundant resources and use environmentally friendly manganese, thus attracting much attention as a positive electrode active material as an alternative to LiCoO2. However, these lithium manganese oxides have disadvantages such as low capacity and poor cycle characteristics.
On the other hand, lithium nickel oxides such as LiNiO2 are not only cheaper than the cobalt oxides, but also higher in discharge capacity, when charged at 4.3V, wherein the reversible capacity of doped LiNiO2 approximates to about 200 mAh/g which is higher than the capacity of LiCoO2 (about 165 mAh/g).
Accordingly, despite slightly low average discharge voltage and volumetric density, commercial batteries including LiNiO2 as a positive electrode active material exhibit improved energy density, and a great deal of research for developing high-capacity batteries using these nickel-based positive electrode active materials is thus being actively conducted. However, despite the advantage of high capacity, lithium nickel oxides have a limitation in practical use because lithium nickel oxides encounter several problems such as a rapid phase transition in a crystal structure caused by volumetric changes accompanying charge/discharge cycles, resultant particle fracture or pores at grain boundaries, generation of a large amount of gas during storage or cycles, and sharp decrease in surface chemical resistance when exposed to air and moisture.
Accordingly, lithium transition metal oxides, in which nickel in the oxides is partially substituted with other transition metals such as manganese and cobalt, have been suggested. Such metal substituted nickel based lithium transition metal oxides advantageously have relatively excellent cycle characteristics and capacity characteristics; however, when used for a long time, cycle characteristics are drastically deteriorated, and problems such as swelling caused by gas generation in a battery and low chemical stability have not been sufficiently solved. Therefore, it is necessary to develop improved techniques to solve problems of high temperature stability while using a lithium nickel based positive electrode active material suitable for a high-capacity battery.
Moreover, lithium nickel based positive electrode active materials basically generate a large amount of lithium by-products (Li2CO3 and LiOH) on the surface thereof. These lithium by-products form a resistive film, and react with a solvent (for example, PVDF) to cause gelation of slurry during the preparation of positive electrode active material slurry, and also generate gas in a battery to cause swelling, thereby significantly reducing life characteristics of the battery.
Therefore, various attempts have been made to solve aforementioned problems through surface stabilization using surface treatment, doping or the like or improvement of structural stability, but efficient methods have not yet been developed.
Based on the above-described background, while conducting research on a method for improving life characteristics of a battery by improving structural stability and by reducing lithium by-products and thus preventing swelling and resistive film formation caused by the by-products, the present inventors found that lithium by-products were significantly reduced on the surface of a positive electrode active material, and life characteristics of a battery including the positive electrode active material significantly increased, wherein the positive electrode active material is prepared in such a way that an alkaline earth metal having oxidation number of +2 was doped into a lithium-nickel based transition metal composite oxide and a phosphate coated layer was formed on the surface of the composite oxide; and finally completed the present invention.